This invention relates to apparatus for the removal of ambiguity in distance measurements in a radar system, and in particular to a continuous wave frequency shift key (FSK) radar.
Radar based distance measurement using FSK are suitable for applications requiring good range discrimination while being capable of manufacture at low cost. One such application is the use of radar of an automotive vehicle for an adaptive cruise control (ACC) related function. The radar detects the distance between a host vehicle fitted with the ACC system and a target vehicle which precedes it and the ACC maintains the vehicle at a safe distance from the obstacle by signaling the engine control or braking control systems of the vehicle.
In any radar-based system, a signal is sent out by an emitter fitted to the vehicle which is reflected from an obstacle back to a detector fitted to the vehicle. The time of flight of the reflected signal provides a measurement of distance. Measuring the Doppler shift between the transmitted signal and the reflected signal over time allows the relative velocities of the host and target vehicles to be determined.
The time of flight can be calculated by counting the number of cycles of the transmitted signal that have occurred between transmission and receipt of an echo. For the relatively high frequencies used in radar and large path lengths for vehicle guidance systems that may be up to 1 km or more, this is impractical as the number of cycles would be very high. Also, if signals are sent continuously it would not be possible to identify how many cycles have passed.
The FSK radar system provides an alternative scheme. A signal comprising a short period of at least two differing frequencies is transmitted which will generate an echo that also comprises two bursts of these differing frequencies. The relative phase of the transmitted and echo signal at each frequency is then determined. By comparing the two phase differences a distance value can be determined over a wide range of distances. The absolute number of cycles is now no longer significant. The differing frequencies are transmitted repeatedly (each group of frequencies being known as a “frame”) and distance measurements are made from each frame.
In one known arrangement, a single transmitter is used to transmit a carrier signal of set frequency which is modulated to form a repeating sequence of frames. Each frame consists of a sequence of steps: each step being a continuous signal of a given frequency.
A measurement of the echo signal is made at the same point in each step, typically towards the end of each step, and compared to the phase of the transmit signal for that step. The phase measurements made for each step within a frame are then compared to determine the range.
With increasing target distance the transmit/echo signals will drift out of phase until such a point that they differ in phase by Pi radians. The two transmit/echo signals will now be in phase. As distance increases, they will again drift out of phase.
It is clearly apparent that once a target vehicle is at a distance from the radar system which is greater than or equal to the distance corresponding to a 360 degree phase shift between all the frequencies of the transmit signal and an echo signal the distance can no longer be unambiguously resolved.
Using more than two different frequencies can provide an increase in the distance over which range can be resolved unambiguously although this brings an increase in the cost and complexity of the device.
One problem of FSK radar systems is that for more distant targets the echo signal that corresponds to a single burst of frequency within a frame may be received during the period of transmission of a subsequent frequency step in that frame. This makes a phase comparison impossible, and so an error signal must be raised to indicate that the distance cannot be determined. Of more concern is the case where the echo from a burst of frequency in a frame is received during the transmission of that same frequency in a subsequent frame. Since this occurs for all steps in the frame (which will all be shifted in time by one complete frame or more) then it is impossible to tell which frame has produced an echo. Not only that, but it is not possible to tell that such an error has occurred.
One known solution to this problem is to increase the length of each frame but this reduces the rate at which distance measurements can be made. Also, it will reduce the rate at which samples can be taken for use in determining the Doppler shift since only one sample can be taken per frame. It is therefore felt that an increase in frame duration is undesirable.
An alternative solution to the problem of ambiguity is to take into consideration the amplitude of the returned signal. It is seemingly reasonable to expect the strength of an echo returned from a near target to be much greater than for a target that is much farther away. Very weak echo signals could therefore be rejected. Unfortunately this only works when the objects that are to be detected are of similar cross sectional area. A lorry at a long distance will probably send back the same amount of signal as a motorcyclist at a closer distance. Some ambiguity can therefore still be present.